The invention relates to a drive unit for a motor vehicle having a hybrid drive and, more particularly, to a drive unit for a motor vehicle having a hybrid drive, including an internal-combustion engine, an electrical rotating machine with a pertaining energy accumulator, a transmission unit, and a control unit.
Many different hybrid vehicle concepts are known, which have an internal-combustion engine and an electrical rotating machine as drive sources. From the trade journal Auto Motor Sport, Edition 11/2004, Page 62, et. seq., “Fahrbericht: Tausendsassa” (“Driving Report: An Allround Winner”, a driving report relating to a BMW X5 is known. Here, a vehicle of the above-mentioned type is described which, in a lower rotational speed range of up to approximately 3,500 rpm, accomplishes the drive by a combination of the conventional internal-combustion engine and an electrical rotating machine. By adding up the torques of the two drive assemblies, a torque of 1,000 Nm is already provided here in the lower rotational speed range at 1,000 rpm. Only when 3,500 rpm have been reached, will the electrical rotating machine be uncoupled from the transmission line, and the drive is implemented in the conventionally known manner by way of the internal-combustion engine exclusively. However, it is contemplated in the case of such systems that, in the event of identical driver intentions or load demands at points in time with a clearly different charging condition of the energy accumulator, a different (not reliably reproducible) starting torque is available to the driver.
It is an object of the invention to indicate a drive unit for a hybrid motor vehicle which, if possible, always provides an identical starting torque, irrespective of the charge condition of the energy accumulator for the electric drive.
According to the invention, this object is achieved by a drive unit for a motor vehicle having a hybrid drive, including an internal-combustion engine, an electrical rotating machine with a pertaining energy accumulator, a transmission unit, and a control unit. The transmission unit includes at least one starting gear position. The control unit is constructed such that, as a function of a charge condition of the energy accumulator, above a defined charge threshold, the starting operation takes place in a standard gear position and, in an event of a charge condition below the defined charge threshold, the starting operation takes place in the starting gear position. The starting gear position has a shorter ratio than the smallest standard gear position.
By constantly monitoring the charge condition of the energy accumulator for the motor drive of the electrical rotating machine and adjusting the transmission ratio as a function thereof for generating the driving torque acting upon the driving wheels, the same driving torque is always provided almost independently of the charge condition of the energy accumulator while the load demand is identical. The energy accumulator for the electrical rotating machine, which is present in addition to a conventional starter battery and is, preferably, constructed as a capacitor arrangement, is monitored with respect to a charge threshold. Should there be a falling below this defined charge threshold, and in the event of an existing current charge condition, a corresponding load demand be present which cannot be met, for example, only by the internal-combustion engine, the transmission unit will be set to a lower gear ratio by way of a control unit, by way of which gear ratio the corresponding load demand (demanded load moment) can be met.
For this purpose, the transmission unit according to the invention has at least one additional gear position which is activated exclusively when the charge condition falls below the predefined charge threshold and exclusively during the starting operation. As an alternative to an additional gear position in the conventional transmission unit, it is also contemplated to provide a separate front-mounted auxiliary transmission with a corresponding ratio and triggering possibility. Particularly, the functionality of an continuously variable front-mounted auxiliary transmission would meet the demands for providing a constantly identical starting torque for the same load demands. With respect to the invention, it is only essential that, as a function of the charge condition of the energy accumulator, the starting operation takes place in a standard gear position of the first ratio or a starting operation takes place in the starting gear position, the starting ratio being smaller than that of the standard gear position. It is, thereby, achieved that a reproducible starting behavior is ensured for any load demand at any time, irrespective of the charge condition of the energy accumulator.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.